Rotate rectangular element 90 degrees on a grid

Question

I'm making a webapp where you can place elements on a grid, move them around and rotate them.

I've run into a problem regarding the rotation of the elements, if the width is an odd number and the height is even and vice versa.

When rotating around the centre of the element, the new coordinates for the element will end off being in halves.

The rotation happens when im adding a class to the element using jQuery, and the actual rotation is made by CSS.

I need to get the element to line up with the grid after a "faulty" rotation. How can i achieve this?

Answer 1

As you are using CSS to rotate your element, you could also use the tranform origin property to make the rectangle rotate around one of the grid nodes and stay aligned. For your example with a 5*2 rectangle the transform origin should be :

transform-origin: 40% 50%;

DEMO (Hover the rectangle to rotate it)

*{box-sizing:border-box;margin:0;padding:0;}
.b{
    float:left;
    width:9.5vw;
    height:9.5vw;
    background:#262C33;
    border:1px solid grey;
}

.r{
    position:absolute;
    left:19vw; top: 28.5vw;
    width:47.5vw; height:19vw;
    border:2px solid #fff;
}
.r:hover{
    transform-origin: 40% 50%;
    transform:rotate(90deg);
}

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